Currently, the primary process for producing ethylene and propylene is the steam pyrolysis, wherein the commonly used raw materials are naphtha. However, there are several shortcomings for steam pyrolysis of naphtha, e.g. high reaction temperature, harsh process conditions, high requirements on the devices, particularly on the furnace tube materials, and high-loss. Various meaningful studies thus have been carried out, in which catalytic cracking is the most attractive and promising. The object thereof is to find a suitable cracking catalyst so as to increase the selectivity to ethylene and propylene, to decrease the reaction temperature and to make the cracking catalyst have some certain flexibility to the ethylene and propylene yield.
From the current known reference documents, most catalytic cracking researchers generally use molecular sieves having a high silica to alumina ratio as catalytic materials and use high valent metallic ions for exchanging and impregnating. However, these molecular sieves have a disadvantage of bad hydrothermal stability and are difficult to regenerate.
U.S. Pat. No. 6,211,104 and CN1504540A disclosed a catalyst consisting of 10-70 wt % of clay, 5-85 wt % of inorganic oxides and 1-50 wt % of molecular sieves. When applying to raw materials conventionally used in steam pyrolysis, said catalyst exhibited excellent activity stability and high yields of light olefin, especially ethylene. Said molecular sieves were produced by impregnating 0-25 wt % of Y type zeolite having a high silica to alumina ratio or ZSM molecular sieves having MFI structure with phosphorus/alumina, magnesium or calcium, and substantially belonged to pure molecular sieve catalysts.
In addition, oxides are also used as catalysts.
U.S. Pat. No. 4,620,051 and U.S. Pat. No. 4,705,769 of PHILLIPS PETROLEUM CO (US) disclosed using an oxide catalyst having manganese oxide and iron oxide as active ingredients and adding rare earth element La and alkaline earth metal Mg, to crack C3 and C4 raw materials. Under the circumstance that Mn, Mg/Al2O3 catalyst was placed in a fixed-bed reactor at a temperature of 700° C. in the laboratory, water and butane were in a molar ratio of 1:1; the butane conversion rate may be up to 80%; and the selectivities to ethylene and propylene were 34% and 20% respectively. Said patents also alleged that naphtha and fluidized-bed reactors could be used therein.
CN1317546A of ENICHEM SPA (IT) disclosed a steam cracking catalyst having a chemical formula of 12CaO.7Al2O3. Naphtha may be used as raw materials. The reaction was carried out at a temperature of 720-800° C. and under 1.1-1.8 atmospheric pressure, and the contact time was 0.07-0.2 s. The yield of ethylene and propylene may be up to 43%.
USSR Pat1298240.1987 disclosed placing Zr2O3 and potassium vanadate on pumice or ceramic into a medium-size apparatus. The reaction was carried under the following conditions: a temperature of 660-780° C., a space velocity of 2-5 hour−1, a weight ratio of water/straight-run gasoline of 1:1. Normal alkane C7-17, cyclohexane and straight-run gasoline were used as raw materials. The ethylene yield could be up to 46%, and the propylene yield could be up to 8.8%.
CN1480255A disclosed an oxide catalyst. By using naphtha as raw material for producing ethylene and propylene via catalytically cracking at a temperature of 780° C., the ethylene and propylene yield may be up to 47%.
Naphtha further contains a part of components having different molecular diameters and different cracking performances, such as aromatic hydrocarbons, cyclanes and the like. The molecular sieves shall have a better selectivity to ethylene and propylene so as to crack these complex raw materials into ethylene and propylene. In order to reduce coking and to decrease the partial pressure of the raw materials, the catalytic cracking should be generally carried out in the environment in which water vapor is present. In addition, the catalyst needs to have a better hydrothermal stability and can be regenerated repeatedly.
Since the molecular sieves such as ZSM-5 molecular sieves, Y zeolites and Mordenitee (MOR) have better shape selective catalytic performances and thermal stability, they are widely used in the petrochemical field. These molecular sieves have a homogeneous pore diameter and have different catalytic performances for the same reactants, so that the separate application thereof is disadvantageous to the processing of raw materials having complex ingredients.
The mechanical mixtures containing these two molecular sieves have multistage pore diameters, and the catalytic performances of the ingredients are different from each other. Thus, when the complex feedstocks having different molecular diameters are processed, the respective catalytic effects of said ingredients can be exerted so as to show better catalytic performances than the single ingredient. However, the mechanical mixtures merely involve the simple mixing of two molecular sieves, and their acid amount and acid strength are also the simple addition of two ingredients. Moreover, their pore diameters do not affect each other; their catalytic effects are separated from each other; the catalytic reaction is finished within each molecular sieve.
In conclusion, molecular sieves as the primary cracking catalysts are attached great importance. However, the examples regarding the co-grown molecular sieves mixing with oxides are rarely reported.